Recently, CERN Courier interviewed Eduard Witten, a master of mathematical physics. Witten talks about the problem of "naturaless," as if this were no longer the principles that would guide physics in the future. He also mentioned the possibility of discovering clues to new physics, and that there are still many possibilities in both experimental and theoretical terms. In the past 50 years, particle physics and cosmology have changed dramatically, what theories are most interesting to him? Talk to the masters and don't miss out.
Interviewer | Matthew Chalmers
Respondents | Edward Witten
Translate | Liu Hang
Leader Edward Witten Professor Charles Simone of the Princeton Institute for Advanced Study 丨Education: B Lacombe/Breakthrough Prize
Edward Witten, regarded as one of the greatest physicists of our time, has worked for nearly 50 years in the frontiers of theoretical and mathematical physics. In this interview, he talks about how the Large Hadron Collider (LHC) and other recent research has influenced his view of nature and explores whether "naturalness"[1] remains the guiding principle in the field.
In physics, naturalness means that the theory of physics has the property that the ratio between free parameters and physical constants should be at the first order level, and that free parameters do not need to be finely adjusted. In this sense, a satisfactory theory should be "natural." It is a standard that stems from physicists' exploration of the Standard Model and deals with broader topics such as hierarchy problems, fine tuning, and anthropic principles. Moreover, it does tend to suggest that current theories,such as the Standard Model, may have weaknesses, some of which vary by many orders of magnitude and require extensive "fine tuning" of the current values of the relevant model. In the history of particle physics, the principle of naturalness has given the correct prediction three times— in the case of electron self-energy, poor π meson mass, and poor K meson mass.
How did the discovery of the Higgs boson in the Standard Model (SM) affect your view of nature?
The discovery of the Higgs boson of the Standard Model is a great triumph of the theory of reformable fields, especially in terms of simplicity. By the time the Large Hadron Collider began operation, the Standard Model, which does not contain a basic Higgs field— such as the dynamic mechanisms of mass generation— has become too complex. It turns out that, as far as we know so far, the original idea of the fundamental Higgs particle was correct. This also means that "nature" chose the theory that encompasses all possible realignment fields— the fields of spin 0, 1/2, and 1— and the flexibility they allow. The Minimum Standard Model is the simplest known model of the Higgs mechanism, containing only one Higgs field, and a model of a double or triplet state of a Higgs particle is also possible.) ）
Another key fact is that the Higgs particles arise on their own, and there is no mechanism that can explain why the energy scale of the weak interaction is so small, while the hypothetical energy scales of gravity, grand unity, and cosmic expansion are much larger. From the perspective of our generation of particle physicists (and I would say, probably not just our generation), it's a pretty big shock. Of course, we experienced a similar shock more than 20 years ago, when we discovered that the universe was expanding at an accelerating rate — the simplest explanation being a very small and positive cosmological constant, the energy density of the vacuum. In at least in both cases, the idea of "naturalness" that seems to accompany us to grow up seems to disappoint has failed.
What do you think of the new methods of solving the "fine-tuning" problem, "relaxion"[2] and "Nnaturalness"[3]?
The method of "relaxion" relaxation is to make the weak interaction field dynamically obtained, and this field evolves and eventually stops at a certain equilibrium point. The main approach to "Nnaturalness" is to introduce N copies of the Standard Model, each with a different Value for the Mass Parameter of the Higgs particle.
For dark energy and hierarchy problems, it is difficult to find a conventional and logical explanation. Reluctantly, I think we have to seriously consider the anthropic principle. According to this theory, we live in a universe full of possibilities that are realized in different regions of space, or in different parts of the quantum mechanical wave function, and we necessarily live where we can live. I don't know if this interpretation is correct, but it provides a measure of other scenarios. 20 years ago, the anthropic principles of the universe disturbed me, probably in part because it made it difficult to understand physics. Over the years, I have experienced more. I think I reluctantly accept the fact that the universe was not created for our convenience in understanding it.
At present, what experimental directions should physicists prioritize?
It is extremely important to delve into the twin mystery of the accelerated expansion of the universe and the small size of the electroweak energy scale, which can test whether our interpretation of the facts is correct and whether it is possible to discover new levels of structure. In the case of the accelerated expansion of the universe, we want to measure the parameter w (the ratio of pressure to energy) as precisely as possible. If the acceleration of the expansion of the universe is controlled only by a simple cosmological constant, then w is equal to -1, but in most alternative models, it will be greater than -1. In particle physics, we want to explore deep structures as accurately as possible, either indirectly, such as the precise study of the Higgs boson, and by further increasing the energy of the collider for direct detection.
Beyond the energy of the Large Hadron Collider, what might be lurking?
If it is possible to eventually enter a higher energy scale, I can imagine several possible outcomes.
It is clear that the traditional idea of "naturalness" is not the whole story, and we have a "naked" Higgs particle in our hands, without the mechanisms that explain its mass. Or, we might find that this apparent failure of "naturalness" is actually an illusion that extra particles and forces can explain the electroweak energy scale, which we don't see now because they are just beyond the scope of our current experiments. There's also an intermediate possibility that I find interesting. That is, the electrodependent energy scale is not natural in the customary sense, but the extra particles and forces can help us understand what happens when the energy exceeds the LHC energy by not too much. A fascinating theory of this type is the "split supersymmetry" proposed by Nima Arkani-Hamed et al.
However, there is an obvious pitfall here. It's easy to say that "one thing or another happens when the energy doesn't exceed too much LHC energy". But in actual experiments, the energy is 3 times LHC energy, 6 times LHC energy, 25 times LHC energy, or more, and this will be a world of difference. In theories such as split supersymmetry, our existing clues are not enough to give a real answer. We very much hope that we can get a concrete clue from the experiments, that is, the energy scale of the new physics beyond the Higgs particle.
Is it possible for a perverse taste to provide some clues?
New leads can come from multiple sources. In CERN's heavy-taste b physical experiments, the anomalies observed are significant, if true. It is also important to look for electron or neutron electric dipole moments, which may give a new physical signal, and under the energy scales we have already observed. Another possibility is the cause of a slight discrepancy between the magnetic moments of μ and the Standard Model predictions. I think it is important to continuously improve the accuracy of the lattice gauge theory for the calculation of the magnetic moments of μ, especially the contribution of the hadron part, to test whether the very accurate measurements that are available today are really inconsistent with the results of the Standard Model. Of course, there are many other aspects. For example, at the next new experimental energy scale, the Standard Model may need to be corrected, from the precise study of the Higgs particle to the search for abnormal decay patterns of μ that do not exist in the Standard Model. These may provide new clues.
Six-dimensional Calaby-Yau manifold. Calabi-Yau manifolds are important for superstring theory. In the most conventional superstring models, there are ten conjectured dimensions in string theory, in addition to the 4 space-time dimensions we know, plus some kind of fibrosis, the dimension of fibers is 6. Witten et al. found that the tightening of Calabi-Yau manifolds is important because they keep some of the original supersymmetry undetermined. 丨Education: A J Hanson
Which of the current theoretical advances is most exciting to you?
The new ideas about gravity and quantum mechanics are generally referred to as "it from qubits" (Editor's note: See Wen Xiaogang on the New Revolution in Physics: Everything Originates from Quantum Information | The Gate of All Wonders"), which is truly exciting. The thermodynamics of black holes was discovered in the 1970s through the work of Jacob Bekenstein, Stephen Hawking, and others. The results are fascinating, but it seems to me that the development of this field for decades – rightly or wrong – has been slow compared to other areas of theoretical physics. However, in the last decade or so, its field has developed rapidly. To a large extent, this change comes from viewing "entropy" as a microscopic or fine-grained von Neumann entropy, rather than thermodynamic entropy considered by Beckenstein et al. The formula for fine-grained entropy makes new and more general formulations possible. When thermodynamics are effective, these formulations can degenerate into traditional formulations. These developments stem from the insight into holographic duality between gravity and gauge theory.
How is the field different now compared to when you first entered the field?
It would be no exaggeration to say that the field has changed dramatically. In September 1973, I started graduate school at Princeton. A few months before that, David Gross, Frank Wilzcek, and David Politzer had just discovered the asymptotic freedom of non-Abelian gauge theory. This is the last key element needed for the Standard Model as we know it today to be possible. Since then, our understanding of Standard Model experiments has revolutionized. At that time, several key components—quarks, leptons, and higgs particles—were unknown. Injection in the hadronation process is also just an idea, let alone an experimental implementation; nothing is known about CP destruction in electroweak theory and scale destruction in quantum chromodynamics, and I have mentioned only two areas of rapid development later here.
Not only is our knowledge of Standard Model experiments much richer now than it was in 1973, but we are even more theoretically understood. The understanding of quantum field theory today is much better than it was in 1973, and there is really no comparison.
Our understanding of cosmology may have changed just as dramatically. In 1973, cosmological knowledge could be summed up well in a few numbers—primarily cosmic microwave temperature and hubble constant. Of these numbers, only the first number was measured with reasonable precision. Subsequently, cosmology gradually became a precise science, but also a more ambitious science, because cosmologists have learned how to deal with the complex processes of the formation of the structure of the universe. In the heterogeneity of the microwave background, we can observe the seeds of structural formation. Compared with the cosmological framework understood in 1973, the theory of the expansion of the universe developed from 1980 can be regarded as a real progress, although it is indeed incomplete.
Finally, I would like to say that 50 years ago, the gulf between particle physics and gravity seemed insurmountable. There is still a great distance today. But string theory, which gives a framework for studying the unity of gravity and particle forces, changes that. This framework has proven to be very powerful, even if not to explain gravity, but only to find a new understanding of quantum field theory. We don't yet know today how to unify these forces, how to get the particles and interactions we see in the real world. But we do have a rough idea of how it works, which is a big change from where we were in 1973. The exploration of the framework of string theory has led to a series of extraordinary discoveries. The well hasn't dried up yet, which is one of the reasons I'm optimistic about the future.
Of all the many contributions you've made to particle and mathematical physics, which one makes you most proud of?
I am most pleased with my work with Nathan Seiberg on electromagnetic duality in quantum field theory in 1994, as well as my work on the condensation and constraint of unipolars in N=2 supersymmetric Young Mills theory[4]), and my work the following year on constructing the correlation of string theory.
Who knows, maybe I'll have the privilege of doing something equally important in the future.
Resources
[1] https://en.wikipedia.org/wiki/Naturalness_(physics)
[2] https://arxiv.org/pdf/1610.02025.pdf
[3] https://arxiv.org/pdf/1607.06821.pdf
[4] https://arxiv.org/abs/hep-th/9407087
[5] https://arxiv.org/abs/hep-th/9503124
This article is translated from Witten eflects, CERN Courier, original address: https://cerncourier.com/a/witten-reflects/
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